carry coals to Newcastle

Newcastle upon Tyne (in England) was a major coal-exporting city, so sending coal there would be pointless.

• "However curious it may seem for an oil-ship to be borrowing oil on the whale-ground, and however much it may invertedly contradict the old proverb about carrying coals to Newcastle, yet sometimes such a thing really happens; and in the present case Captain Derick De Deer did indubitably conduct a lamp-feeder as Flask did declare."
• "He's the fellow who likes to let off stink bombs in night clubs, which rather falls under the head of carrying coals to Newcastle […]"

• bring owls to Athens
• bring sand to the beach
• sell ice to Eskimos

• busman's holiday